Bunching means for particle accelerators



Nov. 19, 1957 M. cH'oDoRow BUNCHING MEANS FOR PARTICLE ACCELERATORS Filed Dec. 16, 1954 United States Patent() BUNCHING MEANS FOR 'PARTICLE A'CCELERATORS Marvin -Chodorow, Menlo Park, Calif., assignor to The Board of Trustees of the Leland Stanford Jr. University, LStanford University, Calif `ApplicationDecember 16, 1954, Serial No. 475,806

13 Claims. (Cl. S15- 5.42)

This invention relates ,to particle accelerators of the linear type. .It is particularly adapted to electron accelerators, but Vthe Vprinciples and practices involved are also adapted to the acceleration of heavier particles, and although the major portion ofthe description which follows will refer to electron accelerators, ,those skilled in the `art will'understand the modifications and adaptations ynecessary to apply the principles to heavy-particle accelerators.

In apparatus of the type under consideration a stream of `the particles to be accelerated is injected into a .wave guide or equivalent structure which is so loaded that the phase velocity of waves propagated therethrough is as low as or lower than the speed of light. Characteristically, such a structure takes the form of a disc-loaded wave guide. This is a tubular conductor having annular, Vinternally-projecting flanges spaced therealong at accurately determined intervals. .The phase velocity of radioV frequency waves set up in such a guide depends upon vthe spacing, the thickness, and the diameter ofthe central yopenings in the annular flanges, these factors being computed for the specific frequency at which the wave guide is intended to be operated. The structure is so designed ,that at its input end the velocity of wave-propagation within the guide is substantially equal to the velocity of the `.particles injected therein. This usually `means that inthe initial portionLof the guide, where the particles are injected, the phase velocity is materially lower thanthat of light, increasing along the length of the .structure to some final value. If the particles injected are electrons, .thelinitiah variable-velocity portion of the guide may be relatively short, the electrons achieving substantially the velocity of light within a foot or so of their point of injection.

YIt will be seen that the structure as above described could equally well be` defined as a succession of coupled cavity resonators. In lcertain types of apparatus the l'at ter description, or a description as a single, highly complex, resonant cavity, might appear to be more `recognizable than the description as a wave guide. The latter description, however, is convenient and will be the one adopted throughout this specification.

Whatever physical form the device may take, high-frequency waves are established at the end 4of the device at which the particles are injected, these waves being established in a mode wherein there is a longitudinal component` ofthe electrical field to which the particles are the phase of the oscillations transmitted by it such that the longitudinal field is in the direction of propagation of the particles and is at or near its maximum, are constantly accelerated, gaining in energy as they travel along the length of the guide. Where the particles are electrons the gain in velocity becomes very 4small after the initial portion of the guide has been passed, the energy gained being effective very largely as an increase in mass.

Particles entering the structure yout of phase with the Acrests of the radio frequency -waves therein are very subjected. Those particles which `enter the structure in mice largely wasted. During one-half of the wave the accelerations applied by the electric field decrease the energy of the particles vinstead of increasing it. The result is that at least half of the particles are lost entirely, while the energy imparted to others, which do make their 4escapelfrom the structure, may vary over a relatively wide range.

At `'the frequencies employed in apparatus of this kind 'it is difficult so to control the emission of the particles that they are emitted from their source, whatever it may be, in bunches, coincident with the crests of the accelerating waves. It has been customary, therefore, in past practice, to inject the particles as a continuous stream and vaccept theloss of particles and the waste of power thatsuch loss involves. In electronic accelerators at least a partial relief from this situation is obtained by so designing .the initial proportion of the guide that it acts as a combinedaccelerator and buncher. This itself involves a loss, since .the phase of the radio-frequency wave at which optimum bunching effect occurs is displaced by .from that resulting in maximum acceleration, and therefore efciency in both bunching and acceleration is sacrificed by accomplishing bunching in the accelerator itself. The accelerator proper is heavy and bulky, and it must be supplied with high power in order -to perform its intended function. The resistance loss in the device is substantially proportional to the power supplied to it. There is, of course, 1a constant attenuation of waves transmitted along the loaded wave guide, and hence, even if .the bunching zper se absorbed fno power, the mere length of .the guide devoted to the relatively inefficient bunch- `ing portionresu-lts in a radio-frequency power loss as well as ina wasteful use of the beam electrons.

Broadly, the purpose of the present invention is to provide means for ,injecting electrons into the wave guide portion of the accelerator with a distribution in phase leading to maximum :utilization of the electrons injected, .thus .to provide .a minimum proportion of wasted electrous, to obtain a minimum dispersion in energy inthe `electrons emergentfrom the guide and to give a maximum .amount Vof energy to the emergent electrons. More specifically, among the Aobjects of the invention are to provide 'means` for injecting into the loaded wave Yguide constituting the major rportion of the device electrons hav- .ing a minimum dispersion in phase; to provide means for injecting previously bunched electrons into the guide in proper phase to receive maximum acceleration in their :passage through the guide; to provide a means for bunchingelectrons injected into a wave guide which is operative at low power and high eiciency; to provide a wave guide which will give the emergent electrons maximum venergy for `ajgiven length of waveguide; and to provide a linear ,accelerator which is efficient both as to the use of the radio-frequency -power supplied thereto in producing ac- `oeleration and in the percentage of the total number of electrons injected .therein which receive such acceleration.

Considered broadly, the electron accelerator of this .invention comprises, in `addition toa loaded wave guide of. .thef general type already described and an electron gun which is .electrically alined with the wave guide for generating. a beam of electrons, so as to direct the beam axially therethrough, Vof a resonant cavity surrounding the path of electrons from the gun, followed by a drift tube, also surrounding the electron path, interposed between the gun andthe accelerator-guide. A radio frequency source or driver supplies energy at a relatively low level to the resonant cavity. Means are provided for supplying the guide with radio frequency energy of the same frequency, this preferably comprising a radio frequency amplifier driven by the source and having its output coupled into thewave guide, and means for adjusting the relative phase to the4 energy lfed to both the cavity resonator and the wave guide. Preferably there is also provided at least one additional cavity, supplied by the same source, interposed along the electron path between the drift tube and the wave guide, together with means for adjusting the phase of the energy supplied to the additional cavity or cavities. In operation, the electron gun supplies a continuous, unmodulated electron stream. As this stream passes through the first cavity the electrons are subjected to either an acceleration or a deceleration, depending upon the phase of the cavity oscillation at which they enter. This first cavity, at least, is preferably made of the reentrant type, so that the maximum electrical field, to which the electrons are subjected, is developed across a very short gap and there is little change in phase during the transit of the electrons of the stream across the gap. Leaving the buncher the electrons pass into the drift tube where they are subjected to neither acceleration nor deceleration. The length of the drift space is so related to the intensity of the eld applied to the electrons in passing through the cavity that at the output end of the drift tube the electrons which first enter the cavity at the beginning of a cycle have been slowed down to such a degree that the electrons of average velocity have caught up with them, while those entering the cavity in the latter part of the cycle have been accelerated to catch up with the average velocity electrons.

The drift tube can terminate directly in the acceleratorguide. Preferably, however, they first pass through at least one additional resonant cavity which is supplied with radio frequency energy in proper phase to accelerate the bunched electrons. The electric field to which the electrons are subjected in the second cavity can be materially higher than that provided in the buncher, and if properly positioned and phased substantially all of the electrons entering the accelerating cavity can do so in an accelerating rather than a retarding phase. Furthermore, by proper adjustment of both phase and relative field strength the electrons which were most retarded in the first cavity can be those most accelerated in the second. Additional drift space may be provided between the second cavity and the accelerator proper, to allow for some further bunchingr of the electrons, but because of their higher velocity in this region the space occupied by the drift tube may be more valuable than the advantage gained from it, and hence it may be omitted. In a preferred form of the invention, therefore, the electron gun operates at a relatively low potential, in the neighborhood of 1000 volts. The buncher cavity is supplied with a radio-frequency oscillation which will develop a field strength across the gap in the cavity of the order of one or two hundred volts. One or more additional cavities, operated at the same or approximately the same effective voltage, may be supplied if desired, and if properly phased additional buncher cavities will improve the bunching, i. e., supply electrons to the final or accelerator cavity more nearly in the same phase. The accelerator cavity is supplied with higher voltage radio-frequency, so phased that the bunched electrons pass through the accelerating gap `at the peaks of the cycle and are accelerated to an energy of somewhere in the neighborhood of 80,000 electron volts, giving them approximately half of the speed of light. This increases their velocity approximately nine-fold, so that the electrical wavelength within the beam leaving the accelerator cavity is nearly nine times as great as in the drift ,tube between the cavities. The beam from the accelerator cavity then passes into the main accelerator, either directly or through a drift tube, as above mentioned. The main accelerator guide is so designed that in its initial portion the phase velocity of the waves propagated therein is substantially the same as the velocity of the electrons in the entering beam, and increases progressively along the guide until it is substantially equal to'that of light and is uniformly of this value throughout the body of the guide.

The invention will be more clearly understood from the following description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings wherein:

Fig. l is a drawing, partly schematic and partly in block form, of an embodiment of the invention wherein a single bunching cavity resonator and one accelerator cavity are provided ahead of the loaded accelerator-wave guide; and

Fig. 2 is a similar diagram illustrating an embodiment of the invention utilizing two bunching cavities and an accelerator cavity as input to the accelerator Wave guide.

The embodiment of the apparatus illustrated in Fig. l illustrates the invention in an embodiment adapted for the acceleration of electrons. The portion of the apparatus with which the invention is particularly concerned comprises an electron gun including an electron emitting cathode 1 associated with one or more accelerating anodes 3, cathode and anode being enclosed within an evacuated glass envelope 5. The gun is sealed to a reentrant cavity resonator 7 comprising a cylindrical body 9 with reentrant tubes 11 and 11 extending inwardly from the ends and separated by a small gap 13. The tube 11 is a continuation of a drift tube 15, which continues or is extended into a second cavity resonator 17, which may be substantially identical with the cavity resonator 7, and its parts are therefore identified by similar reference char acters, distinguished by accents. It may, however, be desirable to make the gap 13 slightly wider than the gap 13. In the present case a short tubular connection 19, substantially similar to the drift tube, connects the cavity 17 with the main body of the electron accelerator.

The electron accelerator proper can be of a type similar to that described in the copending application, Serial No. 450,181, filed August 16, 1954, of Ginzton and Elderidge, now Patent No. 2,761,828. It comprises a tubular wave guide 21, of the disc-loaded type, the phase velocity and the group or energy velocity of the waves propagated therethrough both being decreased by loading discs 23, spaced along the guide at accurately determined intervals. The tube 19 connects into the guide through a small open ing 25 in an input coupling cavity 27. In accordance with known practice, the initial portion of the wave guide 21 is tapered; that is, in this portion the loading discs 23 are spaced more closely together than in the body of the guide and the diameters of the central openings therein are smaller, both the spacing and the opening diameter being increased progressively, from disc to disc, until the uniform portion of the guide is reached. In the particular accelerator illustrated, in connection with which the present invention was originally employed, the length of the accelerator-guide is approximately six feet, the major portion being uniform. Accelerators of the same general character have, however, been built which are several hundred feet long, and the invention is equally applicable irrespective of whether the length of the accelerator is six feet, and the energy imparted to the electrons in traversing it is, say 30 million electron volts, or whether it be several hundred feet long and the electron energy is of the order of magnitude of a billion electron volts or more.

Whatever the length of the accelerator guide, it may terminate in a coupling cavity 29, substantially similar to the input cavity 27. This cavity may, if desired, be provided with a Lenard window 31 in line with the electron beam, to permit the egress of the latter, and any desired equipment for the use of the hunched beam can be appended. On the other hand, particularly for the shorter type of accelerator, a target suitable for the emission of X-rays may be permanently sealed to the device and evacuated together with the portions of the structure already described. The present invention is not concerned with the ultimate use of the accelerated electrons.

For exciting the device thus far described, there is provided a radio-frequency oscillator 33, such as a klystron or magnetron, for generating oscillations in the microwave range. The oscillator 33 connects to a system of radio-frequency transmission lines, preferably of the wave guide type, Transmission line 35 connects to a phasevuntil the instant when'the pulseofler`rci'tatiorr teirintes,

A branch line 43, cenneetedtetween die'phase shifter 37"and the attenuatr '39,. connects" th'ro'gli a'second phase-shifterlS' into the"acceleratorcvity' 17throu`g'l1' the' 5 iris' 47. Another'transinissinline"49"ciinets' directly from the oscillator 33 to a radio-frequency power-ampl tier 51, which'supplieshigH-p'ower energy aittlie q'lency ofthe source 33 through' a'waveguide"53``andiiisSSt the inputfcoupler 27 of 'the ac'ce atr-"gu e.

Connecting into"the"couple"2 a theo'tp the accelerator, throgha'matchin in 57a`nd`w`av` 59is a 1ad`67 for absorbingtlier "frquent" enjer y reaching theoutputend-ofthe loaded-w V'e`g1i' ei" Ith s already been mentioned thattheloadigwliclidecreases' the phase-velocity-withinthe guidealso/"dereassthe en; velopeor energy velocity'rto`a"still"grater`e3 perhaps one percent'offtliefvelocityofflightl' Ii' radiofre-r quency energy fiom the4v source"`"33i`s nF'prviddFcntinuously, but ispulsed, the'length oftlepulses' being n greater th'anftwice thetime ofpropag'a n` ofwavesfffin" the input cavity 27I to the output"cu'i'tyl'29,` ani/"waves reflected from the outputend \wil1not"reach`the-'cavity 27 and'therefore willhave no'eiectuponitlieapprent ifn'l 5 pedance into which the amplifierl 5"1:is connected. Fui'-v tlie'rmore, in a guide of any, materialwlength, the reiected waves alternately accelerate andfd'ecelerate theelectr'ons toequal degrees, Iand'th'erefore lhave no'ill'etfctfupon"tlie over-all operation ofthe yaccelerator,.and `hence it"is.not 30 strictly necessary to provide a loadwTnor-the coupling cavity which connects it tofthe guide iffpulsed operation isvused.` Thispo'rtion of the set-upis 'therefo'reoptional Several types of'equip'ment are available for use vas the phase Shifters 37 -and 45;' They'may, v*fr example, be offthe slottedwave guide type illustratedfinMicrowave Transmission Circuits .(Ragan, vol.-9`,.R'adiation Labbratory Series, McGraw-HilL1948,A page l1'13l)',`.or` they may bel-ofthe dielectrictype illustratedlin, the same work at page5l4. The attenuator 3`9'may comprise ailosser ele-v ment introducedinto thewave guide; Severa1-attenators, suitable for use in connection wit-hitthe piesentlinvention, are illustrated and Adescribed yin *Technique lof Microwave rl\leasurements;livol. v1 1 4ofthe Radiation' Laboratory` Series above cited, pages `748 andffollowing. I 0

inthe operation of ther-invention thusdescrib`ed -the entire combination ofl the electron-gun, .buncher' cavity, drift -tube,,-accelerator cavityy and` the loaded'i'wave lguide 21 are evacuated; they maybe"eitherfsealed"A ot, or coiltinuously pumped,4 .dependingaupon Vtheservice for fwhich the/*accelerator is used, Inoperationdt is convenient toi ground'all portions ofthe equipment except'thev-cathode ofthe-electron gun. The latter is-maintained-,hduringtopf eration, at a potential in ltheneighborhoodof1000 volts negative to ground, -by asuitabl'e source 69` indicatedschematicallyas a battery; l

Operated-at this relatively low voltagelthe electron gun develops Ja continuous, unmodulated beam Wof Aelectrons whichrpasses axially down the enti-redength ofthe 4apparatas-from =thle gun to the window 31,.` lnvtheirvtransitthrough the gap 13 the'electrons'constitutingthebearn are` subjected to the eld between the ends of thelreentrant tubesfll, and`ftheir` velocities are either` increased yduring the positive orf decreasedduringthe*-negative? halfcycles :of the radio frequency eld across-thecgapt`v posi-1 tive?V in this \connectiorr'being'defined' as `the polarity whichV causes an increase in" velocityl'of theelectrons'. Th'epeakf valuelof .the voltage across the g'ap, .forf'a @gun voltagef'of 1000i volts, is preferably in the neighborhood jof forri'tvvo toy threefhundred' volts; and the vl'ci'tyfo'theelectrons ofthe "beam issuflicientlj high? and the* gapissdshot thatffthere yis lith-e2 changefin tHepotenaaracrss-L the 5--gap` dnngf-theiri'stant or transiti After p'ssingthughvtn gap the electrons enter-fthe tdi-itt tribe115?attfelcities-whichv 7?;

CIL

frequency energy exciting thecavity.

Itcan be shownl thatbest' bunchng occurs when where nis the'lengthv of thedrift tube vinwavelel'gt ",*as above' defined, V1 is` thevpealcvoltageacrossethe 'a'pM13f andfVz'is the direct Volta-geb'etween the cathode anl they anode`3l- If n-'equals 2-tlie optiinlnvvalue ofVi-lis 293 volts,Y whereas if n equals 3 the' optiniirvalueI of V1 is 195 volts. The/'values involved are'ln'ot cri't'iczal;t however',v and errors'in'Vthe value ofVi of-cor'siderable'l magnitude can be tolerated.' `Quite satisfactory launching is' obtainable lif "the value of the expression giveriabove lies anywhere between, say, one and two, ratherthanthe' 2" theoretical eXactvale of' 1.845

The value of n is determined by the length ofthedrift tube, the 'velocitiesimparted'to the electron bythe elec.- tr'o'n gunl and the frequency-ofthe?oscillator 33. Ordinarily the en-tire'accelera'tor is designed tofoperatefat'aspecific frequency, and this frequency, taken'irfconju'r'i'cl tion with? the'accelcrating potential which it`Y is'desired to use on the electron gun, will determine the'length of theidrift tube.A

The gap 13 in the accelerator cavity 1'7-'is "positioned the designed number-of wavelengthsfntalong thedrift'ttub'e from they gap 13, so that it is positionedwher'e best bunch# ing occurs. With a* given design,` Whereirllthis'int'ended position'of best bunching is established,l theatteniitor 39-is adjusted to such value that the'bestbunching occu's at the'gap, so that although the length of' the d'rifttube determines the general range in which the potentials est'ah lished across the bunching gap 13 'sh'ould`lie,' inoperation there 'is a considera-ble degree of latitude, through ythe Eadf-l justm'ent of this att'enuatoi'.r

The excitation of'the accelerator lcavity `17 is at lai ni'uclr higher level than that'of the cavity 7, the peakvluelof the fldfacross vthe gap 13 being, in thepresent case, around 80,000 volts. This would'usually dictate lthat"`thegap" 13 be materially longer than the gap 13 to avoid break# down, but otherwise the resonators may bel substantially si'nular. The phase shifter 45'Ais so adjusted that 'the'pek potential across the gap 13 is developed in' the/epochs: when the bunched electrons are traversingthegap. Asi suiningthat the majorityof electrons traverse the gap '13 atthefpeak voltage, this will raise their energy from' 1000 tol 81,000 electron volts," and disregarding the-'relativistic increaselin mass of the electrons, will imparti to therna velocity approximately nine times their'average velocity'in" traversing the drift tube. Assuming'a peak voltage of 200 across the bunchergaplS, the velocity"oftheielectrons' which have traversed the gap 131` will lie in the rangefbie tween" irq/80,800 and K\/s1,2oo meters per secnd where K=594,`500. Takingthe relativity factorfinto-coiil. sideration' the velocity will be about 13% less,'b`u'fth dif-` ference in the velocities ofthe emergent electrons 'vfn the'gap 13', assuming that lthey all pass throughthe'gp" at approximately the peak potential would be, therefrr o'nly'va small fraction of one percent.Y v

The'probability, of cou'rse, isthat the' assumptilon'iasto 0 accuracy of bunching just made is not correct, and tli dispersion in velocity and phase will be greaterffthar.thatI assumed, but'the -velocity diierences will 'stillfbe'srnall".i Becauseof the much greaterv wavelength, of the' electn bunches passing through the drift space 19, atrnch'greate length-? would be --reqired in this regionI to-Jacciplish asians@ either additional bunching or unbunching, v Since it is always desirable to conserve space and bulk in an apparatus of this character, and since bunching after the electrons have been accelerated to the 80,000 volt value would require a long drift tube to have any material eiect, the portion 19 of the drift space is usually made short, so the electrons entering through the orifice 25 in the coupler 27 maintain substantially the same bunching that they had upon passing through the gap 13. In practice the phase shifter 45 is set for the best or highest value of output energy, the final adjustment being made by cut and try to cause the bunches to enter the accelerator proper in phase with the positive crests of the waves in the guide.

On entering the coupler 27 the electrons are immediately subjected to the accelerating iield provided by the amplifier 51, and from this point on the device operates in substantially the same manner as is the case in a conventional accelerator, where the electrons enter as a continuous stream. The difference, however, is that the phase shifter 37 is so adjusted that the already formed bunches of electrons are all subjected to the maximum accelerating iield, and the tapered portion of the loaded wave guide can therefore be made considerably shorter than in the case. of the conventional type of accelerator where a continuous, unmodulated beam of electrons is injected into the loaded guide.

From what has been indicated above it should be evident that the maximum bunching effect is obtainable with relatively slow electrons and that bunching is most elective in the epochs of the cycle where the accelerating lield is changing most rapidly; i. e., when it is passing through zero. This, however, is precisely the time when there is no acceleration of the electrons, and where the bunching is obtained within the major portion of the accelerator it is therefore highly inefficient. In effect, where electrons are injected as a continuous stream, those which enter the loaded guide during the portions of the cycle close to the accelerating peak are accelerated and enter into the useful output of the device, whereas those which enter in epochs f low or reverse potential are almost entirely wasted. Some bunching therefore occurs in the conventional type of accelerator, but this is a highly inefficient process. Initial bunching, as effected by the apparatus here described, can therefore raise the effectiveness of a particle accelerator of this type by several fold in comparison with the conventional type, assuming an equally prolilic source of electrons in the gun. Furthermore, the electrons entering the accelerator in the wrong phase for acceleration absorb power, which is wasted as heat.

The principal parts of the embodiment of the device shown in Fig. 2 may be identical with those shown in Fig. l. These parts are identified with the same reference characters as those used in Fig. l and will not be again described. The apparatus differs from that of Fig. l, however, in that two bunching cavities are used in advance of the accelerator cavity. The drift tube 15, instead of terminating in the accelerator cavity, terminates in a second bunching cavity 71, the drift tube in this case being usually made somewhat shorter than in the form tirst shown. The cavity 71 is supplied with microwave power from the source 33 through a branch wave guide 73, a phase shifter 75 and an attenuator 77. Following the cavity 71 electrons passing through the gap 13 pass down -a drift tube 79, through the accelerator cavity 17 and thence into the connection 19 to the coupler 27 and the loaded wave guide, where the major acceleration takes place, as before.

By properly phasing the excitation of the second buncher 71, the already bunched electrons passing through the gap 13" may have their velocities additionally varied, so that their bunching is materially improved, and they enter the accelerator cavity and the accelerator-guide proper with still less dispersion in phase than where a single buncher was employed.

. Additional bunching resonators can be used withV further improvementin. operation. Whether the additional complication involved in the provision of such additional cavities warrant it depends on the requirements of the service for which the accelerator is to be employed. From the description already given the arrangement of such additional cavities should be apparent those skilled in the art.

The major advantage of the invention is, of course, the much higher effectiveness of the device in the utilization of the electrons suppliedto it. It has, however, other decided operational advantages. Bunching of the electrons at relatively low velocities greatly shortens the apparatus for a given output electron density and energy. Since the electron gun is operated at low voltage, standard types, such as are used in ordinary cathode-ray tubes, may be employed. The lower D. C. voltage makes the operation of the apparatus simpler and safer. The bunching may be made as sharp and as accurate as desired.

The connections through which the various portions of the device are excited are those which have proved, in practice, to be the most convenient, but it should be evident that various other arrangements are readily possible. The use of relatively low power driver oscillator 33 for the bunching and pre-acceleration appears to be the logical one, but it is quite possible to use a high power oscillator, and by proper arrangement of phase Shifters and attenuators to derive the excitation for all of the cavities from this single oscillator. The positions of the various phase shifters and attenuators may be rearranged in various ways to accomplish the same results as the arrangements shown. Such modifications should be self evident to those skilled in the art, however, and need not be described in detail.

Obviously it is also possible to use only the buncher cavity and drift tube and to inject the electrons directly from the drift tube into the loaded wave guide at low velocity, accomplishing all of the acceleration above the initial, 1000 volt acceleration, in the main accelerator. The use of the additional accelerator cavity, however, not only permits the addition of the present invention to conventional already existing accelerators designed for high velocity injection, but it permits the initial acceleration to be given in a relatively high impedance, high voltage cavity, which does not have to be matched to the impedance of a continuous guide, but may be kept electrically separate and thus raise the electron velocity to somewhere in the neighborhood of one-half the speed of light in a shorter distance and more efliciently than in the loaded guide. The latter operates best on high-velocity electrons. Compared to the fields which can be developed in a single cavity those in the accelerator are of relatively low intensity, and the device derives its effectiveness because the electrons are subjected to acceleration therein vcontinuously over a long distance instead of instantaneously, in a narrow gap. In the preferred combination using a separate initial accelerating cavity, the functions of bunching initial acceleration, and linal, high energy acceleration can all be effected at high eciency.

It will be seen that,rtherefore, the invention is not limited to the particular embodiments shown, and these showings are not intended to be construed as limitations upon the scope of the invention. All intended limitations are expressed in the following claims.

l. In combination with a linear particle accelerator comprising a loaded wave guide and a source for supplying radio frequency power to said guide, means for supplying a density-modulated beam of particles to said guide comprising a gun-adapted to produce a constant-density beam, a cavity resonator surrounding the path of said beam, means for exciting said cavity resonator with radio frequency oscillations at the frequency of said source to velocity-modulate the beam passing therethrough, a drift tube surrounding the path of the velocity-modulated beam .to permit bunching of the velocity-modulated particles, a second cavity resonator surrounding the path of the 9 i hunched particles, means for exciting said-second cavity resonator with radio frequency oscillations of the frequency-of said source, said gun, cavity resonators and drift tube being electrically alined with said loaded wave guide to direct particles therethrough, and means for relatively phasing the .oscillations exciting said cavity reso- .:a-torszand said wave guide to cause said second cavity 4I"'esona-t`orran'dsaid wave guide toimpart maximum accel eration to said bunched particles.

2. A particle accelerator comprising an evacuated envelope within which are included an electron gun adapted to produce a constant-density electron beam and means for modulating said beam and accelerating the electrons thereof comprising a first cavity resonator, a drift tube, a second cavity resonator and a loaded wave guide, each surrounding the path of said beam and mounted in succession therealong, and separate connections through said envelope for supplying radio frequency power to said cavity resonators and said wave guide.

3. A particle accelerator comprising an evacuated envelope within which are included an electron gun adapted to produce a constant density electron beam and means for modulating said beam and accelerating the electrons thereof comprising a first cavity resonator, a drift tube, a Second cavity resonator and a loaded wave guide, each surrounding the path of said beam and mounted in succession therealong; a relatively low power source of radio frequency oscillations, a power amplifier, connections from said source to each of said cavity resonators and to said power amplifier, and connections from said power amplifier to said wave guide.

4. A particle accelerator comprising an electron gun adapted to produce a constant density electron beam and means for modulating said beam and accelerating the electrons thereof comprising a first cavity resonator, a drift tube, a second cavity resonator and a loaded wave guide, each surrounding the path of said beam and mounted in succession therealong, a relatively low power source of radio frequency oscillations, a power amplifier, connections from said source to each of said cavity resonators and to said power amplifier, connections from said power amplifier to said wave guide, and means in said connections for varying the relative phase of the oscillations supplied to said cavity resonators and said wave guide.

5. A particle accelerator as defined in claim l including at least one additional cavity resonator and drift tube inter posed between said two mentioned cavity resonators for imparting additional components of velocity modulation to said electron beam, means for exciting said additional cavity resonator with radio frequency oscillations of the frequency of said source, and means for relatively phasing the oscillations exciting said additional cavity resonator.

6. In combination with a linear particle accelerator comprising a loaded wave guide and a source for supplying radio frequency power to said guide, means for supplying a density modulated beam of particles to said guide comprising a gun adapted to produce a substantially constant density beam of said particles, a cavity resonator surrounding the path of said beam, means for exciting said cavity resonator with radio frequency oscillations at the frequency of said source to velocity modulate the beam passing therethrough, a drift tube surrounding the path of the velocity modulated beam to permit bunching of the velocity modulated particles, said gun, cavity resonator, and drift tube being electrically alined with said loaded wave guide to direct said beam of particles therethrough, and means for relatively phasing the oscillations exciting said cavity resonator and said wave guide to produce maximum accelerating elds Within said wave guide at the epochs of maximum density of the particles of the beam entering therein.

7. A particle accelerator comprising an evacuating envelope within which are included an electron gun adapted CTI fo produce a constant density el'eetr'nigheam, mesas rsf modulating said beam andac'celerating theelec'tro'ns' ,theref- Yof comprising a cavity resonator, a drift tube,.and a loaded `wave guide, each surrounding the path. of saidbeam and mounted in succession thereal'ong, and separate connections through said envelope for supplying radio frequency power to said cavity resonator and ,said wave guide.

8. A particle accelerator comprising an evacuated envelope within which are included, an electron gun adapted to produce a constant density electron beam and means for modulating said beam and accelerating the electrons thereof comprising a cavity resonator, a drift tube, and a loaded wave guide, each surrounding the path of said ,beam and mounted in succession therealong; a relatively low power source of radio frequency oscillations, a power amplifier, connections from said source to said cavity resonator and to said power amplifier, and connections from said power amplifier to said wave guide.

9. A particle accelerator comprising an electron gun adapted to produce a constant density electron beam and means `for modulating said beam and accelerating the electrons thereof comprising a cavity resonator, a drift tube, and a loaded wave guide, each surrounding the path of said beam and mounted in succession therealong, a relatively low power source of radio frequency oscillation, a power amplifier, connections from said source to said cavity resonator and to said power amplifier, connections from said power amplifier to said wave guide, and means for adjusting the relative phase of the oscillations sup plied from said :source to said cavity resonator and from said power amplifier to said wave guide.

lO. A particle accelerator comprising an electron gun adapted to produce a constant density electron beam and means for modulatng said beam and accelerating the electrons thereof comprising a cavity resonator, a drift tube, and a loaded wave guide each surrounding the path of said beam and mounted in succession therealong, means for exciting said cavity resonator with radio frequency oscillations of relatively low power, means for exciting said wave guide with radio frequency oscillations of identical frequency and of relatively high power, and means for adjusting the relative phase of the oscillations exciting said cavity resonator and said wave guide respectively.

ll. A particle accelerator comprising an electron gun adapted to produce a constant density electron beam and means for modulating said beam and accelerating the electrons thereof comprising a cavity resonator, a drift tube, a second cavity resonator and a loaded wave guide, each surrounding the path of said beam and mounted in succession therealong, means for exciting said cavity resonators and said wave guide with radio-frequency oscillations of identical frequency, and means for adjusting the relative phase and amplitude of the oscillations exciting said resonators respectively and said wave guide.

l2. A linear particle accelerator comprising a gun adapted to produce a substantially constant-density concentrated beam of relatively low velocity particles, means for bunching and accelerating said particles comprising in succession along and surrounding the path of said beam, a cavity resonator adapted to velocity-modulate said beam, a drift tube wherein velocity-modulated particles of said beam tend to bunc'h and a loaded waveguide; and means for supplying radio frequency oscillations of the same frequency to said cavity resonator and said waveguide respectively at relative phases and amplitudes such as to density-modulate said beam by bunching the particles thereof at their point of entry into said waveguide and to accelerate the bunched particles to a relatively high velocity in their passage through said waveguide.

13. A linear electron accelerator comprising an electron gun adapted to produce a concentrated electron beam of relatively low voltage and substantially constant density, means for bunching and accelerating the electrons of said beam comprising, in succession along and surrounding the path thereof a first cavity resonator adapted to velocityexcitation thereof to produce maximum acceleration of modulate said beam, a dn'ft tube, a second cavity resosaid electrons.

nator, and a loaded waveguide; and means for exciting Y both of said cavity resonators and said waveguides with References Cited in the me 0f this Patent electrical oscillations of the same radio frequency and UNITED STATES PATENTS such relative phase and amplitude that electrons of said beam are bunched to substantially the maximum attainge IIS/le l able degree due to their velocity modulation in said rst 2698381 R b rt's""t"1 D W28 1954 resonant cavity at their entrance into said second resonant o e on e a ec cavity and enter said loaded waveguide at the phase of l0 

